Field of the Invention
The invention relates generally to solar cells and, more particularly, to full solar spectrum group III-nitride solar cells having novel and improved ways to separate and collect photoexcited charges.
Background Discussion
Solar or photovoltaic cells are semiconductor devices having p-n junctions which directly convert radiant energy of sunlight into electrical energy. Conversion of sunlight into electrical energy involves three major processes: absorption of sunlight into the semiconductor material; generation and separation of positive and negative charges creating a voltage in the solar cell; and collection and transfer of the electrical charges through terminals connected to the semiconductor material. A single depletion region for charge separation typically exists in the p-n junction of each solar cell.
Current traditional solar cells based on single semiconductor material have an intrinsic efficiency limit of approximately 31%. A primary reason for this limit is that a semiconductor has a specific energy gap that can only absorb a certain fraction of the solar spectrum with photon energies ranging from 0.4 to 4 eV. Light with energy below the bandgap of the semiconductor will not be absorbed and converted to electrical power. Light with energy above the bandgap will be absorbed, but electron-hole pairs that are created quickly lose their excess energy above the bandgap in the form of heat. Thus, this energy is not available for conversion of electrical power.
Solar cells with higher efficiencies can be achieved by using stacks of solar cells made of semiconductors with different bandgaps, thereby forming a series of solar cells, referred to as “multi-junction,” “cascade,” or “tandem” solar cells. Multi-junction solar cells are made by connected a plurality (e.g., two, three, four, etc.) of p-n junction solar cells. Tandem cells are typically formed using higher gap materials in the top cell to convert higher energy photons, while allowing lower energy photons to pass down to lower gap materials in the stack of solar cells. The bandgaps of the solar cells in the stack are chosen to maximize the efficiency of solar energy conversion, where tunnel junctions are used to series-connect the cells such that the voltages of the cells sum together. Such multi-junction solar cells require numerous layers of material to be formed in a complex multi-junction stacked arrangement.
A standard photovoltaic (PV) device is a semiconductor with a planar p-n junction. Solar photons generate electrons in the conduction band and holes in the valence band. The charges are separated by the built in field at the junction and electrons are collected on the n side and holes on the p side of the junction. A modified form of a planar junction PV device is a hetero p-n junction in which p and n side of the junction are made of different materials. For example CdS/CdTe device comprises n-type CdS layer in contact with p-type CdTe absorber layer. In this case, the electrons are transferred into the n-type CdS and holes into p-type CdTe. In both instances, charge separation occurs at the flat junction parallel to the surfaces. The problem can be treated in one dimensional approximation with the only relevant axis perpendicular to the device surface.
In hybrid PV devices two different materials form separate interconnected networks. The materials are not intentionally doped but have different electron affinities and ionization energies. Photoexcited electron-hole pairs are separated at the interfaces with electrons being transferred to the large electron affinity material and holes being transferred to lower ionization material. Electrons and holes are collected with proper metal electrodes on the opposite side of the device. An example of such hybrid cells are dye sensitized cells in which electron-collecting TiO2 clusters are embedded in the hole-collecting liquid or solid electrolyte.
The operational voltage of a standard PV device is related to the band gap of the semiconductor whereas the current depends on the charge collection efficiency which in turn is mainly determined by the minority carrier (holes on the n-type side and electrons on the p-type side) diffusion length. The diffusion length has to be comparable or larger than the thickness of the light absorbing layer. This puts stringent requirements on the quality of the semiconductor material. In poor quality, highly defective semiconductor charge trapping centers can reduce the carrier lifetime and thus also diffusion length resulting in reduced power conversion efficiency.